In bacterial chemotaxis, t3W I membrane receptors are involved. These have N-terminal extracytoplasmic sensing domains connected by helices to C-terminal signaling domains in the cytoplasm. Receptor signaling involves regulation of a protein kinase, CheA. Kinase activation requires the formation of a higher order complex of 6-8 signaling domains per CheA. By EK these active complexes appear to be cylindrical bundles. STEM studies were initiated to try to deternime the stoichionietry of the components.